Secondary organic materials (SOM) in the atmosphere form two distinct types, liquids and jellies, and these airborne particles have begun to react with gases in the atmosphere. In the last 20 years' research and climate modeling, these SOM particles have been assumed to drift as liquids. In a liquid phase, the organic materials would absorb other compounds like ammonia or ozone very easily and then progress through a series of chemical changes, known as chemical aging, to form particles that reflect or absorb sunlight, or form clouds.

Experiments at Harvard using particles of α-pinene SOM and adipic acid have shown that a drop in humidity can send these common aerosols into a jelly-like phase, in which they resist chemical aging almost entirely. The findings may call for a revision of regional and global climate models.

The two particles chosen for this study, α-pinene and adipic acid, are common in the Earth's atmosphere; α-pinene is essentially a scent released by coniferous trees, including pines, hence the name, and adipic acid comes from both anthropogenic sources (such as car exhaust) and natural chemical reactions.

The effect of phase on the chemical reactivity of adipic acid, a type of secondary organic material that is common in the atmosphere. Courtesy of Kuwata Mikinori and PNAS

"Our research provides the first experimental evidence that the chemical aging process of atmospheric particles is limited by phase," says principal investigator Scot Martin, Gordon McKay Professor of Environmental Chemistry at SEAS and in Harvard's Department of Earth and Planetary Sciences. "Solid or semi-solid aerosol particles will only react with other molecules at the surface of the droplet, instead of mixing homogeneously. What this means is that the time scale of important chemical aging processes may be much longer than what is reflected in current models."

In the atmosphere, the particles of α-pinene SOM and adipic acid behave rather like gelatin; in moist conditions, the droplets absorb water and remain liquid. In dry conditions, they solidify without crystallizing.

Lead author Kuwata Mikinori, a postdoctoral fellow at SEAS, compares these semi-solid atmospheric particles to chunks of tofu, another high-viscosity, amorphous solid. "If you pour soy sauce onto a block of tofu at room temperature, the liquid will just sit on the surface of the tofu. It won't sink in. But if you cook tofu in a sauce at a high heat for a long time, the tofu will eventually absorb the taste of the sauce. That's the same kind of effect we're seeing in the atmosphere. Eventually these semi-solid aerosols do blend with other reactants, but it takes a long time, a higher temperature, or enough ambient humidity to encourage a phase change back to liquid."

The researchers selected ammonia as the reactant in their study partly because its nitrogen component is easy to detect using mass spectrometry, but also because of its current environmental relevance. Atmospheric ammonia has been on the increase in the past few years as a byproduct of fertilizer use and livestock farming, and as a result of increasing temperatures.

"For the environment, ammonia is a very tricky compound," says Kuwata. "It neutralizes sulfuric acid, helping prevent acid rain, but its nitrogen component can also fertilize open bodies of water, which can be bad news for ecosystems. When ammonia reacts with SOM, it can form ammonium salts, which are thought to affect cloud nucleation activity, and organic nitrogen, which forms light-absorbing compounds."

Martin and Kuwata conducted their experiments in the Harvard Environmental Chamber, a 5-cubic-meter Teflon bag that hangs from the ceiling of an environmentally controlled laboratory room at SEAS. In trials within this chamber, they recreated various atmospheric conditions while adjusting the humidity, and exposed particles of either α-pinene SOM or adipic acid to ammonia for approximately seven minutes.

Following that exposure, they measured the diameter of the resulting particles and determined the mass and composition of each one to understand the extent of chemical aging that had occurred.

"Our results challenge basic assumptions about the rate of chemical reactions in the atmosphere," says Kuwata. "These results ought to change the way we evaluate the impacts of atmospheric aerosol particles on the climate."